List of Menu bl_idnames? INFO_MT_file_export, etc
Mousing over the base of a menu gives you the bl_idname.



If you right click and select edit source it will open up the code for that space in the text editor


In this file you will find

class INFO_MT_file_export(Menu):
    bl_idname = "INFO_MT_file_export"
    bl_label = "Export"

    def draw(self, context):
        if bpy.app.build_options.collada:
            self.layout.operator("wm.collada_export", text="Collada (Default) (.dae)")
        if bpy.app.build_options.alembic:
            self.layout.operator("wm.alembic_export", text="Alembic (.abc)")


which affirms that it is a Menu class.  The naming convention used by blender for operators and menus is &lt;SPACE&gt;_&lt;TYPE&gt;_&lt;name&gt; where in this case it's the INFO space, is menu type MT and named file_export.

When a blender menu (or operator) is registered it becomes known to bpy.types.  If the naming convention is followed, we can find all menus in the info space, simply by using auto-complete  in the python console.

&gt;&gt;&gt; bpy.types.INFO_MT_
                      add(
                      armature_add(
                      ...
                      file(
                      file_export(
                      file_external_data(
                      file_import(
                      file_previews(
                      ...
                      window(


To get a complete list of all menus, you can use python's built-in dir() function:

base_type = bpy.types.Menu
for typename in dir(bpy.types):
    bl_type = getattr(bpy.types, typename)
    if issubclass(bl_type, base_type):
        print (bl_type)